A conventional radiation imaging apparatus separately incorporates an imaging radiation detector for two-dimensionally detecting radiation transmitted through a human body to generate an image and a radiation automatic exposure controller (AEC) for controlling exposure of radiation incident from a radiation source.
In a typical imaging radiation detector of this type, generally, pixels each comprising a MIS optical conversion element and a switching TFT are laid out in a matrix, and a phosphor layer which converts radiation into visible light is arranged on the radiation incident surface.
FIG. 12 is an equivalent circuit diagram of a conventional imaging radiation detector. FIG. 13 is a plan view of the imaging radiation detector shown in FIG. 12.
Referring to FIGS. 12 and 13, reference numeral 4008 denotes a semiconductor conversion element such as an optical conversion element; and 4007, a switching TFT. The semiconductor conversion element 4008 and switching TFT 4007 constitute a pixel.
The gate electrodes of the TFTs 4007 are connected to common gate lines (Vg) 4001. The gate lines 4001 are connected to a gate driver 4002 which ON/OFF-controls the TFTs. The source or drain electrodes of the TFTs 4007 are connected to common signal lines (Sig lines) 4003. The signal lines 4003 are connected to an amplifier IC 4004. As shown in FIG. 12, optical conversion element driving bias lines (Vs lines) 4005 are connected to a common electrode driver 4006.
Radiation that becomes incident on an object to be inspected passes through the object to be inspected while being attenuated by it. The radiation is converted into visible light by a phosphor layer. The visible light strikes the optical conversion element and is converted into charges. The charges are transferred to the signal line 4003 through the TFT 4007 in accordance with a gate driving pulse applied from the gate driver 4002 and read out to the outside by the amplifier IC 4004. After that, charges that are generated by the optical conversion element and remain there without being transferred are removed through the optical conversion element driving bias line (Vs line) 4005. This operation is called “refresh”.
FIG. 14 is a schematic sectional view showing the layer structure of one pixel area which is formed from a MIS optical conversion element and a switching TFT (at a position corresponding to a line D-D′ in FIG. 13). In this example, the MIS optical conversion element and switching TFT are simultaneously formed.
The MIS fourth embodiment is constituted by a first conductive layer (lower electrode) 4101, first insulating layer 4102, first semiconductor layer 4103, ohmic contact layer 4105, second conductive layer (bias line) 4106, and transparent electrode 4113 (e.g., ITO). The lower electrode is connected to the source or drain electrode of the TFT 4007. The TFT 4007 comprises the first conductive layer 4101 (gate electrode layer), first insulating layer 4102 (gate insulating layer), first semiconductor layer 4103, ohmic contact layer 4105, and second conductive layer 4106 (source and drain electrodes). Each gate line is connected to the electrode layer where the gate electrode of the TFT 4007 is formed. Each signal line is connected to the layer where the source and drain electrodes are formed. Then, a protective layer (e.g., an SiN and organic film) 4118 and phosphor layer 4119 which converts radiation into visible light are formed on the resultant structure.
An imaging radiation detector constituted by combining a radiation direct conversion material conventionally represented by a-Se, a storage capacitor, and a switching TFT has also been put into practical use.
A radiation automatic exposure controller (AEC) which controls exposure for radiation that becomes incident from a radiation source in the radiation imaging apparatus will be described next.
Generally, in a radiation imaging apparatus having two-dimensionally arrayed sensors, an incident radiation dose must be adjusted (AEC-controlled) for each object or every imaging. Conventionally, an AEC control sensor is arranged independently of the imaging radiation detector. A plurality of thin AEC sensors which attenuate radiation by about 5% are separately arranged in front of the imaging radiation detector. Incidence of radiation is stopped on the basis of the outputs from the AEC sensors, thereby obtaining an appropriate radiation dose for imaging. As an AEC sensor, a sensor which directly extracts radiation as charges by using an ion chamber, or a sensor which converts radiation into visible light through a phosphor, extracts the visible light through an optical fiber, and causes a photomultiplier to convert the visible light into charges is used. FIG. 15 is a view showing the imaging radiation detector and radiation automatic exposure controller (AEC), which constitute the conventional radiation imaging apparatus.
However, as described above, when AEC sensors are prepared independently of the two-dimensionally arrayed imaging radiation detectors to adjust (AEC-control) the incident radiation dose, the layout of the sensors poses a problem. Generally, information necessary for AEC is present at the center of an object. If, AEC sensors should be laid out without impeding image sensing by the imaging radiation detectors, AEC sensors that attenuate radiation by only a minimum amount are necessary, resulting in an increase in cost of the entire apparatus.
In addition, there are no sensors that do not attenuate radiation at all. Hence, the image quality inevitably degrades at the central portion of an object, where an image that is very important for diagnosis is obtained. Furthermore, such separately prepared AEC sensors are disadvantageous for size reduction of a radiation imaging apparatus that is portable and can photograph various portions.
Contrary to this arrangement, U.S. Pat. No. 5,448,613 discloses an arrangement in which a second pixel group is arranged in a sensor substrate and driven by a shift register different from that for an image read sensor to detect the integration of signal charges.
However, when this arrangement is simply employed, some of image read pixels are replaced with second pixels. Accordingly, the opening ratio of pixels related to image reading with respect to all the pixels decreases. In addition, lead interconnections must be prepared separately for the first pixels and second pixels. This may complicate the interconnection structure.
There is still room for improvement in the arrangement of the above prior art in association with the pixel layout and interconnection structure.